1. The Field of the Invention
This invention relates to chemical compositions suitable for maintaining corneal curvature resulting from an orthokeratology procedure, and methods for making and applying the chemical compositions to a cornea for retaining the corneal curvature after undergoing orthokeratological therapy.
2. Description of the Related Art
Structure and composition of a human cornea: The cornea is the first and most powerful refracting surface of the optical system of the eye. Production of a sharp image at the retinal receptors requires that the cornea be transparent and of appropriate refractive power. The refractive power of the cornea depends primarily on two factors: its curvature and its refractive index. When the cornea is misshapened or the axial length of the eye is too long or short, or the lens of the eye is functioning abnormally, various vision related problems, such as myopia, astigmatism, hyperopia, or the like, can result. Eyeglasses or contact lenses are necessary to correct the problems. Eyeglasses correct the refractive errors by refracting the light with a lens before it reaches the cornea and to change the angle at which light enters the cornea. Contact lenses correct refractive errors of the eye by replacing the misshapened cornea with a front curve of a contact lens which is calculated to render the eye emmetropic. When the lens is taken off, however, the cornea is still misshapened or defective and refractive errors still remain.
The cornea contains 75% to 80% water on a wet weight basis. Of the remaining 20% to 25% solids, most are collagen, or other proteins, and glycosaminoglycans. Corneal fibrils, which form the skeleton of the corneal stroma, are neatly organized and present the typical 64 to 66 nm periodicity of collagen. The physicochemical properties of corneal collagen, however, do not significantly differ from those of tendon and skin collagen. Like collagen from these other sources, corneal collagen has high nitrogen, glycine, proline, and hydroxyproline contents. In boiling water or acid, corneal collagen is converted to gelatin, and collagen can be dissolved by proteolytic enzymes such as collagenase, pepsin, or papain.
Orthokeratology procedure: Orthokeratology is a nonsurgical procedure to improve refractive errors of the eye, and is an alternative to, e.g., laser eye surgery. Specifically, orthokeratology is a therapeutic procedure to reshape the curvature of a patient's cornea. A conventional orthokeratology procedure involves the use of a series of progressive contact lenses that are intended to gradually reshape the cornea and produce a more spherical anterior curvature. The process may involve the fitting of three to six pairs of contact lenses, and it has traditionally taken approximately three to six months to achieve optical reshaping. This procedure has been proven to reduce or eliminate myopia and astigmatism, hence improving natural vision and producing emmetropia (a state where vision experiences zero refractive error, or where no correction is necessary). Recent improvement in orthokeratology lens designs make it possible to achieve emmetropia much more rapidly. In many cases, this may be accomplished with a single night's wear of a single pair of end result lenses.
A problem with orthokeratology is that reshaped corneal tissue keeps a memory of its original curvature, and tends to relax and return to the original curvature after the lenses are removed. Therefore, when an orthokeratology patient reaches maximum results, retainer contact lenses are prescribed for full-time or part-time wear to stabilize the results. The retainer contact lenses have typically been made of rigid gas permeable material. Orthokeratology patients increasingly wear retainer contact lenses during the night to obtain the desired results quickly, and enjoy almost emmetropic vision during their daytime activities. A disadvantage of such a modality is that it requires the wearing of retainer lenses every night in order to keep the cornea from regressing to its former shape.
Corneoplasty: A related procedure directed to solve this problem uses a corneal softening agent to temporarily soften the cornea so that it can be more easily reshaped to a desired configuration to produce emmetropia. The corneoplasty procedure is a three-step process performed in one visit or over a period of several weeks. The three-step process includes: first, applying the softening agent to the cornea to soften corneal tissue; second, placing a rigid contact lens over the cornea to render the eye emmetropic; and third, applying a stabilizing agent. The cornea would then reshape and conform to the desired configuration dictated by the rigid contact lens. Administration of the corneal softening agent helps correct larger refractive errors in a shorter period of time.
However, it has been found that it is difficult to accurately place the shaping contact lens with respect to the axis of vision to control the reshaping of the corneal tissue. In some unsuccessful applications, corneoplasty has induced astigmatism or double vision due to errors caused by misplacing the shaping contact lens. In addition, because all three steps are performed in one visit, the patient lacks an opportunity to react to the result of reshaped corneal tissue. The patient cannot “try and see” or change his/her mind during the process.
In light of the foregoing, there is a need for an improved method for performing an acuity correcting procedure that enables the patient to quickly reach emmetropia, while retaining the option of reverting back to his prior level of vision, e.g, to make the procedure be reversible up until the patient chooses to have the correction made permanent. Furthermore, there is a need for compositions and methods for making and applying the composition that stabilizes the cornea matrix. The compositions need to be able to stabilize the corneal curvature resulting from the orthokeratology procedure so that an orthokeratology patient can dispense with wearing rigid retainer contact lenses, dispense with applying a softening agent, and yet retain the opportunity to regress to the original corneal curvature up until the patient is convinced that they want the correction made permanent.